The present invention relates generally to improvements in the transferring of digital data of communication networks, and more particularly pertains to a new and improved communication control system wherein data transfer between a data processor and a plurality of modems is controlled.
The prior art to which the invention is directed includes time divisional transferring of data between a plurality of modems and a data processor by way of a communication control system including a plurality of communication lines. The modems are connected to the corresponding communication lines to receive data by demodulating signals on the communication lines and send data to the communication lines by modulating digital data signals from the communication control system.
The communication control system comprises a line control unit and a communication control unit. The line control unit is connected to the modems and selects each one of the modems by scanning them in a fixed order and with a fixed repetition period, connecting the selected modem to common lines. For this operation, the line control unit comprises a plurality of line adapters, each of which is connected to a selected modem, as well as a decoder to select one of the line adapters.
An address signal is applied to the decoder from the communication control unit. The decoder selects one of the line adapters, therefore, one of the modems.
The communication control unit comprises a line memory to store data and controlling means which controls communication with the data processor and the line adapters.
For easy layout, the communication control unit and the line control unit are generally housed in different cases and are mutually connected with cables, which are as long as 50 meters for many applications. Through these long cables, address signals to select one of the line adapters are sent from the communication control unit to the decoder of the line control unit, and then control signals and data, if any, are sent from the selected line adapter to the control means in the communication control unit.
The control means modifies the data read out of the line memory according to the control signal and the data, if any, already sent from the line control unit and then writes the modified data in the line memory and now sends back a control signal and data, if any, to a selected line adapter.
These steps are required for transferring one bit of data between the communication control unit and the line control unit.
The signal propagation time for 50 meter cables is about 250 nsec. For the one cycle of operation mentioned above, 750 nsec is required for signal propagation only. Therefore, the time required for one cycle of operation of transferring one bit of data between those two units cannot be made smaller than 1 .mu.sec. Therefore, according to the prior art, the speed of data transfer between those two units is limited and as a result, for example, the communication control system of the prior art is not fast enough for 8 communication lines of 96 k bits/sec.